Immunoelectron microscopical studies on viruses

Two types of virus-related particles are found during transmissible gastroenteritis virus morphogenesis

The intracellular assembly of the transmissible gastroenteritis coronavirus (TGEV) was studied in infected swine testis (ST) cells at different postinfection times by using ultrathin sections of conventionally embedded infected cells, freeze-substitution, and methods for detecting viral proteins and RNA at the electron microscopy level. This ultrastructural analysis was focused on the identification of the different viral components that assemble in infected cells, in particular the spherical, potentially icosahedral internal core, a new structural element of the extracellular infectious coronavirus recently characterized by our group. Typical budding profiles and two types of virion-related particles were detected in TGEV-infected cells. While large virions with an electron-dense internal periphery and a clear central area are abundant at perinuclear regions, smaller viral particles, with the characteristic morphology of extracellular virions (exhibiting compact internal cores with polygonal contours) accumulate inside secretory vesicles that reach the plasma membrane. The two types of virions coexist in the Golgi complex of infected ST cells. In nocodazole-treated infected cells, the two types of virions coexist in altered Golgi stacks, while the large secretory vesicles filled with virions found in normal infections are not detected in this case. Treatment of infected cells with the Golgi complex-disrupting agent brefeldin A induced the accumulation of large virions in the cisternae that form by fusion of different membranous compartments. These data, together with the distribution of both types of virions in different cellular compartments, strongly suggest that the large virions are the precursors of the small viral particles and that their transport through a functional Golgi complex is necessary for viral maturation.

Intracellular transport of the murine leukemia virus during acute infection of NIH 3T3 cells: nuclear import of nucleocapsid protein and integrase

The entry and intracellular transport of Moloney-murine leukemia virions inside mouse NIH 3T3 cells have been followed by electron microscopy techniques. Five viral proteins--matrix (MA, p15), capsid (CA, p30), nucleocapsid (NC, p10), integrase (IN), and the envelope glycoprotein (SU, gp70)--were located by immunolabeling using gold probes. After entering the cells, viral particles were frequently detected inside cytoplasmic vesicles of variable size. Their viral envelope was apparently lost during intracytoplasmic transport. When the unenveloped viral cores reached the nuclear membrane or its vicinity, they were disrupted. Two of the immunolabeled proteins, NC and IN, were detected entering the nucleus of non-dividing cells, where both were targeted to the nucleolus. However, MA and CA were found only in the cytoplasm. NC is a nucleic acid-binding protein which contains potential nuclear localization signals. We suggest that NC could enter the nucleus as part of a nucleoprotein complex, associated with IN, and possibly, also with viral DNA.

Membrane protein molecules of transmissible gastroenteritis coronavirus also expose the carboxy-terminal region on the external surface of the virion

The binding domains of four monoclonal antibodies (MAbs) specific for the M protein of the PUR46-MAD strain of transmissible gastroenteritis coronavirus (TGEV) have been located in the 46 carboxy-terminal amino acids of the protein by studying the binding of MAbs to recombinant M protein fragments. Immunoelectron microscopy using these MAbs demonstrated that in a significant proportion of the M protein molecules, the carboxy terminus is exposed on the external surface both in purified viruses and in nascent TGEV virions that recently exited infected swine testis cells. The same MAbs specifically neutralized the infectivity of the PUR46-MAD strain, indicating that the C-terminal domain of M protein is exposed on infectious viruses. This topology of TGEV M protein probably coexists with the structure currently described for the M protein of coronaviruses, which consists of an exposed amino terminus and an intravirion carboxy-terminal domain. The presence of a detectable number of M protein molecules with their carboxy termini exposed on the surface of the virion has relevance for viral function, since it has been shown that the carboxy terminus of M protein is immunodominant and that antibodies specific for this domain both neutralize TGEV and mediate the complement-dependent lysis of TGEV-infected cells.

Immunoelectron microscopy of enzymes, multienzyme complexes, and selected other oligomeric proteins

The collective term "immunoelectron microscopy" subsumes a number of techniques in which the biological material is decorated with specific antibodies, prior to being visualized in the electron microscope. In this article, we have reviewed literature on immunoelectron microscopy that focusses on the analysis of the molecular architecture of proteins, in particular of enzymes and of multienzyme complexes. Molecular immunoelectron microscopy has been remarkably successful with multi-subunit enzymes of complex quaternary structures, and in many cases the data have been the basis for the eventual development of detailed three-dimensional molecular models. The elucidation of subunit composition and juxtaposition of a given enzyme, an important accomplishment in itself, has in turn stimulated and guided discussions on the catalytic mechanism; illustrative examples include F1 ATPase and citrate lyase, among others. Here we have chosen a variety of enzymes, multienzyme complexes, and non-enzymatic proteins to demonstrate the versatility of immunoelectron microscopy, to illustrate methodological prerequisites and limitations, and to discuss significance and implications of individual immunoelectron microscopy studies.

Small colloidal gold conjugated to Fab fragments or to immunoglobulin G as high-resolution labels for electron microscopy: a technical overview

Fab-colloidal gold labelling in conjunction with negative staining and high-resolution electron microscopy was used for targeting single protein units in regular arrays. These were bacteriophage T4 polyheads with Fab-Au2.5, and a specific antibody binding site on the haemagglutinin polypeptide of influenza virus with Fab-Au3, Fab-Au2.5, and Fab-Au1-2. For the latter, IgG-Au3 was also used. Experimental details are summarized to provide generally applicable methods for the preparation of small gold colloids Fab-Au and of labelling. The putative mechanism of protein-gold complex formation and adsorption to preferred sites on Fab and IgG, most probably to sulphur-rich regions, is discussed. The influence of pH during complex formation was found to be of minor importance in the samples investigated. Reported experimental details and our own experiences suggest that the importance of a protein's pI relative to its optimum gold complexing pH critically depends on the nature of the protein in question rather than being of general importance for protein-gold complex stability.

Virological applications of the grid-cell-culture technique

Whole mounts of intact virus-infected cells have been used for several decades to examine virus-cell relationships and virus structure. The general concept of studying virus structure in association with the host cell has recently been expanded to reveal interactions between viruses and the cytoskeleton. The procedure permits utilization of immuno-gold protocols using both the transmission and scanning electron microscopes. The grid-cell-culture technique is reviewed to explain how it can be exploited to provide valuable information about virus structure and replication in both diagnostic and research laboratories. The use of the technique at the research level is discussed using bluetongue virus as a model. The procedure can provide basic structural information about intact virions and additional data on the intracellular location of viruses and virus-specific structures and about the mode of virus release from infected cells. Application of immunoelectron microscopy reveals information on the protein composition of not only released virus particles but also cell surface and cytoskeletal-associated viruses and virus-specific structures. Collectively, this simple and physically gentle technique has provided information which would otherwise be difficult to obtain.